VEBTEBEATES. 405 



scarcely to require more minute comparisons than those detailed above 

 to distinguish it. 



Position and locality: Black carbonaceous shales overlying the "Pan 

 ora coal-bed" of the Middle Ooal Measures; Guthrie county, Iowa. 



Genus PETALOBHYKCHTTS, Agassiz. 

 Petalorhynchus psetjdosagittattjs, St. J. and W. 



PI. XII, Kg 1-4. 



Teeth small or of medium size. Crown irregularly pentagonal in 

 outline, moderately thick, sharp-crested, acuminate or broadly rounded 

 between the prominent lateral angles of the convex face; the concave 

 face forms a spatulate area, gently depressed in the middle, lateral 

 angles slightly produced, from which the basal border is profoundly 

 arched downward, the lateral margins gradually converging in a slight 

 curvature to the obtusely rounded inferior border, where the coronal 

 belt is very wide and gently beveled to the base, and composed of seve- 

 ral imbrications, of which the lower and narrower ones follow the curva- 

 ture of the margin, the upper folds gradually increasing in width and 

 more or less angularly arched upward in the middle, where they occupy 

 from one-fifth to one-third of the vertical diameter of the crown, but as 

 they diverge towards the lateral angles of the crown they suddenly 

 narrow, and are seldom present, rarely being even faintly discernible 

 in the present condition of the teeth ; the convex crown-face is rela- 

 tively low, probably in the majority of individuals not more than half 

 the vertical diameter of the opposite face, gently arched laterally, 

 nearly plane vertically, the basal margin forming a more or less promi- 

 nent angle and gently beveled below, produced in the median region 

 into a nearly horizontal shoulder, from the angles of which the beveled 

 imbricated belt arches upward and again strongly curved downward on 

 nearing the acute lateral angles of the crown. The coronal surfaces 

 are invested with a thin enamel layer, which, however, is generally 

 exfoliated to greater or less extent, especially in the convex face, giving 

 rise, by the exposure of the minute tubular structure, to the diverging 

 striation observable in the majority of specimens. The base as seen 

 from the convex side, slightly expands laterally below the coronal bor- 

 der into an obtuse lateral angle, below which the margins gradually 

 converge to the rounded inferior border, the surface gently convex below 

 and rising into a more or less angularly defined ridge above, correspond- 

 ing to the angulation in the coronal margin, from which it is beveled to 

 the lateral margins; the concave aspect presents a short Ungulate pro- 



